I'll elaborate a bit more on ChrisR awesome answer and bring images from his awesome reference.
A valid JSON always starts with either curly braces { or square brackets [, nothing else.
{ will start an object:

{ "key": value, "another key": value }
Hint: although javascript accepts single quotes
', JSON only takes double ones".
[ will start an array:

[value, value]
Hint: spaces among elements are always ignored by any JSON parser.
And value is an object, array, string, number, bool or null:

So yeah, ["a", "b"] is a perfectly valid JSON, like you could try on the link Manish pointed.
Here are a few extra valid JSON examples, one per block:
{}
[0]
{"__comment": "json doesn't accept comments and you should not be commenting even in this way", "avoid!": "also, never add more than one key per line, like this"}
[{ "why":null} ]
{
"not true": [0, false],
"true": true,
"not null": [0, 1, false, true, {
"obj": null
}, "a string"]
}
Answer from cregox on Stack OverflowI'll elaborate a bit more on ChrisR awesome answer and bring images from his awesome reference.
A valid JSON always starts with either curly braces { or square brackets [, nothing else.
{ will start an object:

{ "key": value, "another key": value }
Hint: although javascript accepts single quotes
', JSON only takes double ones".
[ will start an array:

[value, value]
Hint: spaces among elements are always ignored by any JSON parser.
And value is an object, array, string, number, bool or null:

So yeah, ["a", "b"] is a perfectly valid JSON, like you could try on the link Manish pointed.
Here are a few extra valid JSON examples, one per block:
{}
[0]
{"__comment": "json doesn't accept comments and you should not be commenting even in this way", "avoid!": "also, never add more than one key per line, like this"}
[{ "why":null} ]
{
"not true": [0, false],
"true": true,
"not null": [0, 1, false, true, {
"obj": null
}, "a string"]
}
Your JSON object in this case is a list. JSON is almost always an object with attributes; a set of one or more key:value pairs, so you most likely see a dictionary:
{ "MyStringArray" : ["somestring1", "somestring2"] }
then you can ask for the value of "MyStringArray" and you would get back a list of two strings, "somestring1" and "somestring2".
On the outermost level, a JSON object starts with a { and end with a }.
Sample data:
{
"cars": {
"Nissan": [
{"model":"Sentra", "doors":4},
{"model":"Maxima", "doors":4},
{"model":"Skyline", "doors":2}
],
"Ford": [
{"model":"Taurus", "doors":4},
{"model":"Escort", "doors":4}
]
}
}
If the JSON is assigned to a variable called data, then accessing it would be like the following:
data.cars['Nissan'][0].model // Sentra
data.cars['Nissan'][1].model // Maxima
data.cars['Nissan'][2].doors // 2
for (var make in data.cars) {
for (var i = 0; i < data.cars[make].length; i++) {
var model = data.cars[make][i].model;
var doors = data.cars[make][i].doors;
alert(make + ', ' + model + ', ' + doors);
}
}
Another approach (using an associative array for car models rather than an indexed array):
{
"cars": {
"Nissan": {
"Sentra": {"doors":4, "transmission":"automatic"},
"Maxima": {"doors":4, "transmission":"automatic"}
},
"Ford": {
"Taurus": {"doors":4, "transmission":"automatic"},
"Escort": {"doors":4, "transmission":"automatic"}
}
}
}
data.cars['Nissan']['Sentra'].doors // 4
data.cars['Nissan']['Maxima'].doors // 4
data.cars['Nissan']['Maxima'].transmission // automatic
for (var make in data.cars) {
for (var model in data.cars[make]) {
var doors = data.cars[make][model].doors;
alert(make + ', ' + model + ', ' + doors);
}
}
Edit:
Correction: A JSON object starts with { and ends with }, but it's also valid to have a JSON array (on the outermost level), that starts with [ and ends with ].
Also, significant syntax errors in the original JSON data have been corrected: All key names in a JSON object must be in double quotes, and all string values in a JSON object or a JSON array must be in double quotes as well.
See:
- JSON specification
- JSONLint - The JSON validator
A good book I'm reading: Professional JavaScript for Web Developers by Nicholas C. Zakas 3rd Edition has the following information regarding JSON Syntax:
"JSON Syntax allows the representation of three types of values".
Regarding the one you're interested in, Arrays it says:
"Arrays are represented in JSON using array literal notation from JavaScript. For example, this is an array in JavaScript:
var values = [25, "hi", true];
You can represent this same array in JSON using a similar syntax:
[25, "hi", true]
Note the absence of a variable or a semicolon. Arrays and objects can be used together to represent more complex collections of data, such as:
{
"books":
[
{
"title": "Professional JavaScript",
"authors": [
"Nicholas C. Zakas"
],
"edition": 3,
"year": 2011
},
{
"title": "Professional JavaScript",
"authors": [
"Nicholas C.Zakas"
],
"edition": 2,
"year": 2009
},
{
"title": "Professional Ajax",
"authors": [
"Nicholas C. Zakas",
"Jeremy McPeak",
"Joe Fawcett"
],
"edition": 2,
"year": 2008
}
]
}
This Array contains a number of objects representing books, Each object has several keys, one of which is "authors", which is another array. Objects and arrays are typically top-level parts of a JSON data structure (even though this is not required) and can be used to create a large number of data structures."
To serialize (convert) a JavaScript object into a JSON string you can use the JSON object stringify() method. For the example from Mark Linus answer:
var cars = [{
color: 'gray',
model: '1',
nOfDoors: 4
},
{
color: 'yellow',
model: '2',
nOfDoors: 4
}];
cars is now a JavaScript object. To convert it into a JSON object you could do:
var jsonCars = JSON.stringify(cars);
Which yields:
"[{"color":"gray","model":"1","nOfDoors":4},{"color":"yellow","model":"2","nOfDoors":4}]"
To do the opposite, convert a JSON object into a JavaScript object (this is called parsing), you would use the parse() method. Search for those terms if you need more information... or get the book, it has many examples.
Lean towards option 1, as it's a more expected format.
Option 1 works with JSON as it's designed to be used and therefore benefits from what JSON offers (a degree of human readability, which is good for debugging, and straightforward parsing, which is good for limiting entire categories of bugs to begin with).
Option 2 begrudgingly adopts JSON and subverts many of the benefits. If you don't want human readability, use protobuf or something similar... AIWalker's "CSV"-like approach isn't terrible either. It is marginally better (readable) than splitting objects apart and recombining them. But, this is still not as good (readable) as using JSON "as designed".
Also bear in mind, your API responses are also likely going to be gzipped. Most of the repetition in option 1 will be quickly and transparently condensed over the wire.
As an aside, if you're moving a lot of data, also consider JSONL or paginated results. Pagination can be especially helpful for web clients, as it places natural pauses in the processing, providing a degree of "organic" protection against UI lockups.
A list of objects is easier to work with. You can use append, map, filter... All the nice things JS Arrays have which manual indexing doesn't. And there's no way to get out of sync, so that's an entire class of bugs gone.
If you're worried about efficiency:
- Measure (premature optimization is the root of all evil)
- Consider the list of lists trick AIWalker proposed
- Consider an outright binary format
- Make sure gzip is enabled
- Measure (it's worth saying twice)